The present invention relates to a magnetic resonance imaging (abbreviated as xe2x80x9cMRIxe2x80x9d hereinafter) apparatus for obtaining tomograms of desired sections of an object to be examined by utilizing nuclear magnetic resonance (abbreviated as xe2x80x9cNMRxe2x80x9d hereinafter). In particular, it relates to an MRI apparatus capable of distinguishing the direction of blood flow when depicting the travel of blood flow.
One known imaging ability of an MRI apparatus is MR angiography (abbreviated as xe2x80x9cMRAxe2x80x9d hereinafter), which depicts blood flow without using an imaging contrast agent or the like. Typical imaging methods of blood flow using the MRI apparatus include the time-of-flight (TOF) method, which utilizes the inflow effect of blood into slice planes, and the phase method, which utilizes phase dispersion of flow spins caused by gradient magnetic field in the direction of the flow. The phase method includes the phase-sensitive (PS) method and the phase-contrast (PC) method. The PS method depicts blood flow by subtraction of data acquired in a sequence compensating the phase dispersion of flow (rephase sequence) and data acquired in a sequence promoting the phase dispersion to diminish flow signals (dephase sequence). The PC method utilizes a pair of gradient magnetic field pulses (flow encoding pulses) imparting different phase rotation to flow spins, and depicts the blood flow by applying these pulses to obtain a pair of data and performing complex subtraction of these flow-encoded data.
Each of these conventional flow imaging methods has merits and demerits and is employed in accordance with the purpose of imaging. For example, the 2D-TOF method, which performs RF irradiation onto an object region with a short repetition time TR (presaturation), should use a short-TR sequence such as the gradient echo method as its imaging sequence and cannot employ a fast sequence such as EPI. In addition, since signals of flow spins flowing into the region from any direction are enhanced, arterial flow and venous flow cannot be distinguished. A technique of presaturating a region adjacent to the imaging region maybe employed but, in this case, only one of the arterial flow and venous flow can be visualized.
On the other hand, the PC method also provides a technique for distinguishing one of the arterial flow and venous flow from the other by taking subtraction between flow-encoded phase images with different polarities. However, when the flow velocity is high enough to cause a phase shift of more than xcfx80, the PC method produces a problematic aliasing artifact. In addition, since the sequence of the PC method is not of the flow-rephase type, it cannot cope with turbulent flow or variance of the flow velocity, and blackened flow images or artifacts are therefore likely to occur. Moreover, a two-dimension PC method aiming at quantitative measurement of flow velocity cannot thicken the thickness of a slice including blood vessels and is not suitable for observing the overall travel of blood together with quantitative measurement.
Therefore, an object of the present invention is to provide an MRI apparatus having a new ability of blood flow imaging with little artifact and capable of distinguishing the arterial and venous flows (depicting blood flow direction) together with enabling observation of all vessels.
Another object of the present invention is to provide an MRI apparatus capable of employing a fast imaging sequence such as EPI and thereby imaging flow in a short measuring time.
In order to achieve the above object, the present invention provides an MRI apparatus having a function of, for a plurality of slices, performing excitation of a slice by application of radio frequency magnetic field and measurement of echo signals successively, and reconstructing blood images using the obtained echo signals, wherein the apparatus performs, during performing said function, a first measurement of exciting the plurality of slices in a first order and a second measurement of exciting the slices in an order opposite to the first order to obtain two kinds of data for each slice, and subtraction operation between the two kinds of data for each slice to obtain signals from blood flows in different directions as data having different signs.
Specifically, the MRI apparatus of the present invention comprises means for generating a static magnetic field in a space where an object to be examined is accommodated, means for generating gradient magnetic fields to impart a gradient to the static magnetic field, a transmitting system for repeatedly applying radio frequency pulses according to a predetermined pulse sequence to cause nuclear magnetic resonance of nuclear spins of atoms constituting living tissue of the object, a receiving system for detecting echo signals emitted by the nuclear magnetic resonance, a signal processing system for performing image reconstruction operation using the echo signals detected by the receiving system, means for displaying the obtained images, and a controlling system for controlling operations of the gradient magnetic field generating means, transmitting system, receiving system, and signal processing system, wherein the controlling system performs a first measurement of repeatedly exciting the plurality of slices in a first order with a predetermined repetition time and a second measurement of exciting the plurality of slices in an order opposite to the first order with the same repetition time to obtain two kinds of data for each slice, and the signal processing system performs subtraction operation between the two kinds of data for each slice to obtain signals from blood flows in opposite directions as data with different signs.
The magnetic resonance angiography (MRA) method of the present invention is a method of, for a plurality slices, performing excitation of a slice by application of radio frequency magnetic field and measurement of echo signals successively, and reconstructing a blood image using the obtained echo signals, which comprises the steps of (a) performing a first measurement of exciting the plurality of slices in a first order and a second measurement of exciting the slices in an order opposite to the first order to obtain two kinds of data for each slice, and (b) performing subtraction operation between the two kinds of data for each slice to obtain signals from blood flows in different directions as data having different signs.
In the first measurement, where the plurality of slices are excited successively in the first direction (direction A), blood flow in the direction A is repeatedly excited with a relatively short repetition time and signals from the blood flow spins become relatively weak. On the other hand, the repetition time TR of blood flow excitation in the opposite direction (direction B) becomes relatively long and signals thereof become approximately the same as that of static spins. In the second measurement, where the same plural slices are excited successively in the direction B opposite to the first direction, signals from the blood flow in the direction B become relatively weak and signals from the blood flow in the direction A become relatively strong, contrary to the first measurement. Accordingly, when subtraction is performed between data obtained by these two kinds of measurements, the pixel value (signal intensity) of the static part becomes 0 and, for flow spins, the sign of the data differs depending on the flow direction.
The MRA of the present invention preferably further includes step (c) of saturating at least one of region adjacent to the object region by applying a radio frequency magnetic field prior to the step (a) (pre-saturating step).
In the slice positioned at each side of the object region, signals of blood spins flowing into the slice would not be suppressed by multiple excitations and would not be distinguished from those from blood spins flowing out of the slice. However, by pre-saturating the region adjacent to the object region, signals are suppressed by multiple excitations and, therefore, blood spins of different directions, i.e., inflow spins and outflow spins, become distinctive. Thus, ability of imaging blood flow can be improved throughout the object region. Instead of pre-saturating the region adjacent to the object region, data of slices at both sides may be deleted from data to be processed in step (b). This deletion of data should be construed as being within the scope of the present invention.
In the MRA of the present invention, it is also preferred that, in the measurement of each slice, the slice be selected so that it partially overlaps each adjacent slice.
When each slice is excited so that the adjacent slices overlap each other, a static part of the overlapped portion is repeatedly excited and, therefore, signal intensity of the static part can be more suppressed than when slices do not overlap. Thereby, the blood flow imaging ability can be improved.
Gradient echo type sequence maybe employed as the imaging sequence of the present invention and the number of echo signals measured at each excitation maybe one or more. Since blood signals can be depicted with high contrast relative to that of the static part even though the same slice is not excited many times, a multiple-echo sequence (including EPI), which measures a plurality of echo signals everyone excitation, maybe employed.